Add support for self-referenced types

[mattreddy3]

Hi,

Thanks as always for maintaining this project! Wanted to ask for an enhancement. We sometimes reference field types across entity definitions (not necessarily referencing a distinct type declaration). For example:

entity MyEntity {
  Field1: String; // We will reference this field
  Field2: Integer;
}

entity TypeRefEntity { 
  Field3: MyEntity:Field1; // Should resolve to `String` as type
}

Here is an example of where this currently fails:

The error is _a.includes is not a function since _a is not a string as expected.

I'm using cds@6.8.1 - not sure if that is the problem? As a workaroudn I will try to explicitly type these.